Window for gallium nitride light emitting diode

ABSTRACT

A window structure for a gallium nitride (GaN)-based light emitting diode (LED) includes a Mg+ doped p window layer of a GaN compound; a thin, semi-transparent metal contact layer; and an amorphous current spreading layer formed on the contact layer. The contact layer is formed of NiO x /Au and the current spreading layer is formed of Indium Tin Oxide. The p electrode of the diode includes a titanium adhesion layer which forms an ohmic connection with the current spreading layer and a Schottky diode connection with the Mg+ doped window layer.

The present invention relates to an improved window for a galliumnitride (GaN)-based light-emitting diode (LED).

BACKGROUND OF THE INVENTION

A semiconductor light-emitting diode (LED) includes a substrate, a lightemitting region, a window structure, and a pair of electrodes forpowering the diode. The substrate may be opaque or transparent.Light-emitting diodes which are based on gallium nitride (GaN) compoundsgenerally include a transparent, insulating substrate, i.e., a sapphiresubstrate. With a transparent substrate, light may be utilized fromeither the substrate or from the opposite end of the LED which is termedthe “window”.

The amount of light generated by an LED is dependent on the distributionof the energizing current across the face of the light emitting region.It is well known in semiconductor technology that the current flowingbetween the electrodes tends to concentrate in a favored path directlyunder the electrode. This current flow tends to activate correspondingfavored portions of the light-emitting region to the exclusion ofportions which fall outside the favored path. Further since such favoredpaths fall under the opaque electrode, the generated light reaching theelectrode is lost. Prior art GaN LEDs have employed conductive currentspreading layers formed of nickel/gold (Ni/Au), and have a gold (Au)window bond pad mounted on such layers. In such arrangements, the Ni/Aulayer and/or the Au bond pad tend to peel during the wire bondingoperation to the pad.

SUMMARY OF THE INVENTION

In one embodiment consistent with the present invention, light isutilized at the output of the window structure, which includes a verythin, semi-transparent nickel oxide/gold (NiO_(x)/Au) contact layerformed on a p-doped nitride compound window layer; a semi-transparentamorphous conducting top window layer; and a p electrode structureformed of a titanium layer with a covering Au bond pad. The amorphoustop layer, by way of example, may be formed of indium tin oxide (ITO),tin oxide (TO), or zinc oxide (ZnO). Layers of other amorphous,conductive, and semi-transparent oxide compounds also may be suitablefor construction of the top window layer.

Advantageously, the thin NiO_(X)/Au layer provides an excellent ohmicconnection to both the amorphous current spreading conducting layer andto the magnesium (Mg)-doped GaN window layer. The highly conductiveamorphous layer efficiently spreads current flowing between theelectrodes across the light-emitting region to improve the efficiency ofthe device.

Additionally, the titanium electrode passes through both the amorphousconducting layer and the underlying Ni/Au to: (a) form an ohmic contactwith those layers; (b) contact the p-doped top widow layer and form aSchottky diode connection therewith; and (c) provide good adhesionbetween the titanium (Ti) and the magnesiusm (Mg)-doped window layer.The Schottky diode connection forces current from the electrode into theamorphous conducting layer and eliminates the tendency of the prior artstructures to concentrate current in a path directly under theelectrode.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic depicting a cross-sectional view of an LEDaccording to one embodiment consistent with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Figure depicts an LED according to one embodiment consistent withthe present invention, as a GaN-based device in which light exitsthrough window 109.

The LED of the Figure includes a sapphire substrate 101, buffer region102, GaN substitute substrate layer 103, n cladding layer 104, activeregion 106, p cladding layer 107, window layers 108, 109, n electrode105, and a window structure which includes a thin NiO_(x)/Ausemi-transparent layer 110, a semi-transparent amorphous conductinglayer 111, a titanium electrode 112, and a bond pad 113.

Layers 101 through 104, and layers 106 through 109, are grown in a MetalOrganic Chemical Vapor Deposition (MOCVD) reactor. The details of MOCVDgrowth of the stated layers are well known in the semiconductor industryand will not be discussed herein.

The remaining components of the illustrative LED, namely, layersNiO_(X)/Au layer 110, amorphous conducting layer 111, n electrode 105, pelectrode 112, and bond pad 113, are formed by evaporation in anapparatus other than a MOCVD reactor. Such processes are well known inthe semiconductor industry and are not described herein.

The Light-emitting Structure

The illustrative light-emitting structure of the Figure includes an ncladding layer 104, active region 106, and p cladding layer 107.

The n cladding layer 104 is formed of silicon-doped GaN.

In the illustrative example depicted by the Figure, active region 106 isa silicon-doped n-type gallium indium nitridie/gallium nitride(GaInN/GaN) multi-quantum well (MQW) structure. However, other forms ofactive regions may be utilized with the illustrative window structure.

The p cladding layer 107 is formed of Mg-doped aluminum gallium nitride(AlGaN).

The Window Layers

The first window layer 108 is formed of Mg-doped GaN. The window layer108 has a nominal thickness of 300 nm.

The second window layer 109 is similarly formed of Mg-doped GaN.However, window layer 109 is more highly doped to permit an ohmiccontact between layer 109 and the very thin NiO_(x)/Au layer 110.

Completion of the MOCVD Growth Process

Growth of the p-type GaN layers is achieved with the introduction ofgaseous flows of TMG with hydrogen (H₂) as a carrier gas, NH₃ as a groupV material, and Mg as a dopant. In the absence of an appropriate cooldown protocol, hydrogen passivation of the Mg may occur, in which case,the conductivity of the Mg-doped layer is reduced.

In order to avoid hydrogen passivation of the Mg-doped layers 107, 108,and 109, the following described cool-down protocol has been adoptedupon completion of the MOCVD growth.

1. The ambient gas of the reactor is switched from H₂ to nitrogen (N₂)immediately after completion of the LED structure;

2. The reactor temperature is ramped down from the growth temperature toabout 900 degrees C. in about 2 minutes;

3. The flow of NH₃ is terminated;

4. The reactor temperature is further ramped down to about 750 degreesC. in about 2 minutes;

5. A temperature of about 750 degrees C. is held for about 20 minutes;

6. The heater of the reactor is shut off and the reactor is allowed tocomplete cool-down naturally. Experience shows that cool-down to 120degrees C. occurs in about 30 minutes after heater shut off.

The resulting product exhibits the expected desired physical andelectrical characteristics.

Formation of the Electrode Structures

The embodiment consistent with the present invention as depicted by theFigure, illustrates the locations of both p electrode layers 111, 112and n electrode 105.

Layer 110 is a very thin, semi-transparent contact layer of NiO_(X)/Auwhich is deposited over the entire exposed face of window layer 109.Opening 114 is formed in layers 110 and 111 to permit the deposit of atitanium adhesion layer 112 to contact window layer 109. Titanium formsa strong physical bond with layer 109 and thus tends to eliminatepeeling during wire bonding. In addition to reaching through to layer109, titanium structure 112 is deposited through and on top of amorphouslayer 111. Titanium electrode 112 forms ohmic contacts with layers 110and 111, and forms a Schottky diode contact with window layer 109. TheSchottky diode connection to window layer 109 eliminates the currentpath directly under the electrode and forces current flowing between theelectrodes into conducting layer 111.

The p electrode Au bond pad 113 is deposited on top of titanium layer112 to form an ohmic contact.

Since the Mg-doped layers do not suffer from hydrogen passivation, it isnot necessary to heat treat the structure to activate the Mg doping inthose layers. However, Ni/Au layer 111 and the Ti and Au contactstructures are heated in an atmosphere of molecular nitrogen and air.Thus the Ni is converted to a form of nickel oxide. The described heattreatment improves the quality of the contact structures.

The invention has been described with particular attention to itspreferred embodiment. However, it should be understood that variationsand modifications within the spirit and scope of the invention may occurto those skilled in the art to which the invention pertains.

What is claimed is:
 1. A light emitting diode comprising: a substrate; alight emitting region; a window structure; and first and secondelectrodes; wherein said window structure comprises: a semi-transparentmetal contact layer, and a semi-transparent, conductive amorphouscurrent spreading layer formed directly on an exposed face of saidcontact layer; and wherein an opening is formed through said contactlayer and said current spreading layer and said first electrodecomprises a layer of titanium formed on said current spreading layer andthrough said opening to contact an upper surface of said Mg+ dopedwindow layer.
 2. The light emitting diode in accordance with claim 1,wherein said contact layer is a NiO_(x)/Au layer.
 3. The light emittingdiode in accordance with claim 1, wherein said amorphous currentspreading layer is formed of Indium Tin Oxide.
 4. The light emittingdiode diode in accordance with claim 2, wherein said amorphous currentspreading layer is formed of Indium Tin Oxide.
 5. The light emittingdiode in accordance with claim 1, wherein said window structurecomprises: a Mg+ doped window layer; and wherein said Ni/Au contactlayer is formed on said Mg+ doped window layer and said first electrodeforms an ohmic connection with said current spreading layer.
 6. Thelight emitting diode in accordance with claim 5, wherein said firstelectrode forms a Schottky diode connection with said Mg+ doped windowlayer.
 7. The light emitting diode in accordance with claim 2, whereinafter heat treatment, said contact layer comprises a Ni oxide/Au layer.8. A light emitting diode comprising: a substrate; a light emittingregion; a window structure; and first and second electrodes; whereinsaid window structure comprises: an Mg+ doped window layer; asemi-transparent NiO_(x)/Au contact layer formed on said Mg+ dopedwindow layer; and a semi-transparent, conductive amorphous currentspreading layer formed of indium tin oxide directly on an exposed faceof said contact layer; wherein said first electrode forms an ohmicconnection with said current spreading layer; and forms a Schottky diodeconnection with said Mg+ doped window layer; wherein an opening isformed through said contact layer and said current spreading layer; andwherein said first electrode comprises a layer of titanium formed onsaid current spreading layer and through said opening to contact anupper surface of said Mg+ doped window layer.
 9. A light emitting diodecomprising: a substrate; a buffer region; a GaN substitute substratelayer; an n cladding layer; an active region; a p cladding layer; adouble window layer structure; an n electrode; a window structurecomprising: a semi-transparent metal contact layer, and asemi-transparent, conductive amorphous current spreading layer formeddirectly on an exposed face of said contact layer; a titanium electrode;and a bond pad; wherein an opening is formed through said contact layerand said current spreading layer to said double window layer structurefor said titanium electrode.